Various types of printing machine have ink ductor rollers which have doctor blades applied thereto, in which the engagement gap or nip of the doctor blade with respect to the ductor roller can be adjusted across the length of the ductor roller in respective zones. The doctor blade is flexible and, in some zones--distributed axially along the ductor roller--the ink film can be thicker than in others. The adjustment of the relative position of the doctor blade with respect to the ductor roller, which determines the thickness of the ink, can be done manually or automatically. The referenced REES application Ser. Nos. 185,414 and 185,390, both filed on Sept. 9, 1980, now U.S. Pat. Nos. 4,325,303 and 4,314,506 describe such apparatus. Electrical servo motors can be used to adjust the position of the adjustment screws by providing suitable control signals thereto.
Film scanning apparatus in order to determine the areal coverage of a film, particularly a film composite from which an offset printing plate is to be made, has been described in the literature--see German Patent Disclosure Document DE-OS No. 26 18 387. As disclosed therein, a light source is provided which passes light throughout its entire width through the film, the light then being sensed by a suitable receiving apparatus. The receiving apparatus is subdivided into a plurality of zones which are connected to an apparatus to evaluate the sensed values. The signals, so obtained, are then used to control the zone doctor blade adjustment screws in the inking fountain system of rotary offset printing machines. In this film scanning apparatus, the amplitude of the signals obtained by measuring the original in the various sensing zones is subject to different edge conditions and variations. These edge conditions are, for example, regions or zones of lesser light intensity of the illumination source towards the marginal regions, different influences of stray light and stray reflected light, manufacturing tolerances of the photoelectric transducers, and the like. It is important that, with the same object to be printed, and the same coverage relationships, that is, ink distribution relationships, the output signals should always be the same. To obtain this reproducibility of output requires frequent calibration. The apparatus of this type often uses integrators in order to eliminate erroneous readings which, then, have to be readjusted or recalibrated. Recalibration is time-consuming and can be carried out only for one specific element to be measured or sensed with its own specific characteristics of materials.